In Pulse Amplitude Modulated (PAM) digital transmission systems, digital pulses of substantially uniform amplitude and duration are transmitted between terminal facilities by way of regenerative repeaters spaced at intervals along the transmission medium. Each repeater first distinguishes between the presence or absence of a pulse and then regenerates each detected pulse with substantially its original amplitude and duration. Because of the frequency dependent nature of the transfer characteristics of the transmission media, such as a wire cable, it is necessary to provide equalization for the preceding section of cable at each repeater point.
Tarbox, in U.S. Pat. No. 3,568,100 issued Mar. 2, 1971, teaches that the transfer characteristic of a length of cable connecting adjacent regenerator repeaters in a digital transmission system can be approximated by the combination of flat loss and a single attenuation pole. The magnitude of the flat loss and the frequency of the pole are both dependent upon the length and the gauge of the cable. For a given gauge of cable, the flat loss increases with length and the frequency of the pole decreases. For a given length of cable, the flat loss increases and the frequency of the pole decreases as the cable gauge becomes finer.
In the Tarbox equalizer system, the peak signal level at the output of the equalizer is detected and the gain, i.e., amplitude response of the equalizer is varied along with the frequency location of a simple real zero of the equalizer's frequency response. The gain and zero location is varied in accordance with a complex non-linear relationship, which is a function of the detected peak voltage level. For very short cable lengths, i.e., below 10 db loss, the non-linear relationship given by Tarbox does not provide adequate compensation. Therefore, a manual switch must be provided for adjusting compensation parameters when the loss is less than 10 db.
A bi-quad automatic cable equalizer (Bi-Quad ACE) is U.S. Pat. No. 3,824,501 to Harris, issued July 16, 1974, wherein the "eye openings" of signals emanating out of transmission cables is maximized. Equalization is achieved by monitoring the equalizer's peak output signal, and adjusting the gain k, of the equalizer to maintain a constant output signal level, and by altering the frequency location of a simple real zero, g, in the equalizer's transfer response in accordance with the relation 1/k=K.sub.1 g+K.sub.2, where K.sub.1 and K.sub.2 are equalizer constants.
The "eye opening" defines a decision region, within each transmission clock period, that is best suited for performing the signal regeneration task. A more thorough treatment of the "eye diagram" subject is found in the book entitled Data Transmission, by W. R. Bennett and J. R. Davey, p. 119, McGraw-Hill, 1965.
It was determined by C. Harris and W. Farmer, at Bell Telephone Laboratories, that good eye openings, i.e., greater than 70% over a length (l) of cable involving less than 34 db loss, could be achieved using an equalizer which compensated for a cable transfer function T(s) having a loss K(l) and a single pole P(l) with a hyberbolic relationship between K(l) and P(l).